Crude oils are comprised of two major solubility fractions, maltenes and asphaltenes. Maltenes constitute the fraction of oil that is soluble in low molecular mass n-alkane solvents, such as n-pentane, n-hexane, and n-heptane. Asphaltenes are defined as the crude oil fraction that is soluble in aromatic solvents and insoluble in low-boiling straight chain alkanes. Asphaltene molecules have complex structures and are typically polar molecules with relatively high molecular weights (approximately 700 to 1,000 g/mole). Asphaltenes can contain carbon, hydrogen, nitrogen, oxygen, and sulfur, as well as trace amounts of vanadium and nickel.
Asphaltenes are typically stable under virgin reservoir conditions, but can be destabilized and precipitate from crude oil during production due to changes in temperature, pressure, chemical composition, and shear rate. Asphaltene deposits can occur throughout the production system, from inside the reservoir formation to pumps, tubing, wellheads, safety valves, flow lines, and surface facilities used in the extraction process. Asphaltene deposits can cause production rate decline and other operational problems, such as increased fluid viscosity and density, and stabilization of oil-water emulsions. The nature of asphaltene deposits, which can appear hard and coal-like or sticky and tar-like, is determined by the composition of the crude oil and the conditions under which precipitation occurred.
Chemical treatment with additives such as dispersants and inhibitors is one of the commonly adopted control options for the remediation and prevention of asphaltene deposition. In general, asphaltene dispersants (ADs) are nonpolymeric surfactants. The polar and/or aromatic head groups in ADs are thought to interact with aggregated asphaltenes and make them more dispersible in the crude oil. Asphaltene inhibitors (AIs) are typically polymers. AIs provide real inhibition in that they can shift asphaltene flocculation pressure and prevent aggregation of asphaltene molecules. Determination of AI concentration in crude oil production is important for understanding the residence time of polymers down-hole as well as the effectiveness of precipitation reduction.
Techniques such as liquid chromatography-time of flight mass spectrometry (LC-TOF MS) have been used in the past to quantify residual asphaltene inhibitor in crude oil. However, the sample preparation procedure is very time consuming and expensive. In addition, LC-TOF MS may have limitations in use for different types of polymeric asphaltene inhibitors. Thus, an improved method for the detection and quantification of phenolic formaldehyde polymer type AIs in crude oil is still needed.